docs: def _error_dm(self, m, dm, s):
          """Error function.
          Once self.goal has been defined, compute the error
          of input using the generalized forward model.
          """
          pred = self.fmodel.predict_given_context(np.hstack((m, dm)), s, range(len(s)))
          err_v  = pred - self.goal
          error = sum(e*e for e in err_v)
          return error